Question

Why thermosphere temperature increases with altitude?

Answer 1

Short answer - Gravity

Long answer - The gravity causes the upper layers of atmosphere to exert force on the lower ones due to their weight. This causes the air to compress and hence the air become dense at the lower level(closer to surface) while in comparison the air at heights( far from surface) is rarer or thinner.

Dense air means more air molecules per unit volume and opposite for rarer air.

Analogy - Imagine a large number of same sized sponges placed one over another. The one at bottom will be most compressed due to weight of others and this will decrease as we look at the sponges on the top